The present invention relates to fiber optic cables, and, more particularly, to fiber optic cables of the type having at least one strength member.
Conventional fiber optic cables include optical fibers that conduct light which is used to transmit voice, video, and data information. Where the fiber optic cable is subjected to crushing forces, the optical fibers may be stressed and attenuation of the transmitted light may occur. It is therefore important for fiber optic cables to be constructed in a robust manner whereby attenuation due to crush induced stresses can be avoided. Additionally, although it is desirable for a fiber optic cable to have a high optical fiber count, it is also desirable for the cable to be as compact as possible.
Conventional fiber optic cables should be able to withstand longitudinal compression and tension, and they typically include strength members for these purposes. However, the strength members may disadvantageously affect cable bending performance during installation, and may hinder optical fiber access. A fiber optic cable having strength members located in a single plane generally will experience a preferential bending action favoring bending of the cable out of the plane defined by the strength members. On the other hand, a fiber optic cable having strength members at spaced locations encircling the center of the cable will not have a preferential bend, but the strength members typically include a helical lay so that the cable can be bent. Even taking into account the helical lay of the strength members, when bent in generally any axis, cables of the non-preferential bend type may be very stiff, a characteristic which may be highly undesirable depending upon installation requirements. Thus a cable of the preferential bend type will typically experience ease of cable bending in a preferred axis, and, as there are less strength members to deal with, may present a less time consuming optical fiber access procedure. A cable designer may therefore balance the need to have sufficient cable components for resisting crush, compression, and tension loads, against the size and stiffness contributions of the cable components that may render the cable difficult to install in a cable passageway.
An example of a known preferential bend type fiber optic cable is disclosed in U.S. Pat. No. 5,509,097. This known cable requires a hot melt adhesive or epoxy which bonds the strength members to a core tube. The adhesive/epoxy may be combined with a helically wound tape or cord wrapped about the strength members under tension, thereby applying radially inward forces to the strength members and forcing the strength members against the tube. Due to the size and stiffness contributions of the cable components, this known cable may be undesirably stiff for some cable installations. Additionally, the cord, tape, and adhesive/epoxy add expense to the cable product and may complicate/slow the manufacturing process. Moreover, large gaps exist adjacent the strength members between the jacket and the core tube which may negatively affect the crush performance of the cable.
An example of a known non-preferential bend type fiber optic cable is disclosed in U.S. Pat. No. 4,730,894, which is incorporated by reference herein in its entirety. This known cable includes a layer of strength members adhesively bonded to a plastic tape and wrapped about a core tube; however, the cable may be undesirably stiff for some cable installations. Additionally, the plastic tape and adhesive add size and expense to the cable product and may complicate/slow the manufacturing process. Moreover, the lack of a tight coupling between the plastic tape and the core tube may negatively affect the crush performance of the cable. Other non-preferential bend type cables may be expensive and/or undesirably stiff as they include, in addition to a core tube, a helically grooved outer tube having helically laid trapezium shaped reinforcing members bonded thereto. The reinforcing members are made of a material that weakly bonds to the slotted rod.
It is an object of the present invention to provide a fiber optic cable having at least one optical fiber in a tube and strength assemblies adjacent the tube for imparting crush resistance to the cable. The strength assemblies may include at least two opposed strength members imparting a preferential bend resistance to the optical cable, at least one of the strength members being in contact with a first jacket, and being surrounded by an armor tape and a second jacket.
It is another object of the invention to provide a fiber optic cable having: at least one optical fiber in a tube; strength assemblies respectfully comprising at least one strength member in contact with a jacket and adjacent the tube; at least one ripcord generally aligned with a bending axis of the cable and generally in contact with at least one of the strength members; whereby crush loads applied to the cable create stresses therein which flow to the jacket and to the strength members.
It is an object of the invention to provide a fiber optic cable having: at least one optical fiber in a tube; strength assemblies each respectively comprising at least two longitudinally disposed strength members generally in contact with each other; at least a first jacket surrounding the tube, at least some of the strength members generally in contact with the at least first jacket; and at least one ripcord disposed generally between two of the strength members of a strength assembly and generally longitudinally aligned with the strength members.